How to Identify Male & Female Medaka

1 min read

A Beginner’s Guide: How to Tell Male and Female Medaka Apart

If you’ve got medaka (Japanese rice fish) and want to know whether they’re male or female, don’t worry—it’s actually pretty easy once you know what to look for. Here are the four easiest ways to tell the difference.

1. Look at the Anal Fin (Easiest Method)

The anal fin is the fin on the underside of the fish near the tail.

Male medaka

Longer and more pointed
Often has a jagged or uneven edge

Female medaka

Shorter and rounded
Smooth edge

👉 If one fish has a long, pointy anal fin and another has a short rounded one, the pointy one is almost always male.

2. Check the Dorsal (Top) Fin for a Notch

Another reliable way to identify males is by looking at the dorsal fin, which is the fin on the top of the fish.

Male medaka

Often has a small notch or split toward the back of the dorsal fin

Female medaka

Dorsal fin is smooth and rounded
No notch or split

👉 This notch can be subtle, but once you notice it, it becomes an easy feature to spot.

3. Check the Body Shape

Females usually look rounder, especially when carrying eggs.

Male medaka

Slimmer body
More streamlined shape

Female medaka

Slightly plumper
Belly looks rounder

4. Look for Eggs (Dead Giveaway)

If you see small eggs hanging near the vent under the belly, that fish is definitely female.

Female medaka often:

Carry eggs for a short time after spawning
Look like they have little yellow or clear bubbles attached underneath

Breeder Tip: Watch the Fish Interact

Experienced breeders often sex medaka just by watching their behaviour.

During breeding activity:

Male medaka

Chase females around the tank
Swim tightly beside females
Attempt to wrap slightly around the female during spawning

Female medaka

Swim away from chasing males
May appear slower or heavier when carrying eggs

👉 If you see one fish constantly chasing another, the chaser is almost always the male.

💡 Pro tip: Once you learn to spot the anal fin shape and dorsal notch, you can usually sex medaka in just a few seconds.